The US warns Iran it will continue surveillance flights in the region and that the military will protect its assets.

Iranian warplanes fired multiple rounds at an unmanned unarmed US surveillance aircraft in international airspace over the gulf last week, but the craft was undamaged and returned safely to its base, Pentagon spokesman George Little said on Thursday.

US President Barack Obama and Defense Secretary Leon Panetta were both advised early on about the incident, which occurred at about 4:50 a.m. ET on November 1. Iran was later warned that the United States would continue to conduct such surveillance flights in the region and that the military would protect its assets.

According to the Pentagon, two Iranian SU-25 "Frogfoot" aircraft intercepted the American drone as it conducted a routine, but classified, surveillance mission over Gulf waters about 16 nautical miles off the Iranian coast.

If Iran had hit the drone, as the Pentagon believes it was trying to do, it could have forced American retaliation - with the potential consequences that entails.

Pentagon spokesman George Little said the aircraft fired multiple rounds at the Predator drone and followed it for at least several miles as it moved farther away from Iranian airspace.

"We believe that they fired at least twice and made at least two passes," he said.

International airspace begins after 12 nautical miles and Little said the drone at no point entered Iranian airspace. Last year, a crashed CIA drone was recovered inside Iran.

"There is absolutely no precedence for this," Little said. "This is the first time that a (drone) has been fired upon to our knowledge by Iranian aircraft."

Many questions about the incident remain, including why Iranian warplanes could not manage - if they wanted - to shoot down an unarmed drone, which lacks advanced capabilities to outmaneuver them.

Asked whether the Iranian aircraft were simply firing warning shots, Little said: "Our working assumption is that they fired to take it down. You'll have to ask the Iranians why they engaged in this action."